The present invention relates to a heat exchanger having at least two collecting pipes connected with one another by a plurality of small regularly spaced streamlined-section tubes and a method of making same. More particularly, the present invention relates to a heat exchanger which can be manufactured at more reasonable costs.
A heat exchanger having at least two collecting pipes is shown, for example, in DE-PS 36 36 762 where the collecting pipes have a plurality of cutouts for receiving small streamlinedsection tubes. In order to achieve a good rate of heat exchange, it is necessary to provide a very large number of small streamlined-section tubes which may be in the range of several thousand. The disadvantage of these known heat exchangers is that the cutting of thousands of cutouts out of the massive walls of the pipes by electro-chemical processes, requires hundreds of production hours.
A collecting pipe utilizing concentric sleeves with a material layer inserted therebetween is shown in DE-PS 37 22 329. There, a sinterable powder for the formation of the collecting pipe wall is poured between two molding shells. The removal of the molding shells on the streamlined-section tube side disadvantageously entails considerable expenditures. Moreover, because of deviations of the actual density from the theoretically achievable density, hollow spaces may exist through which leakage currents may flow to the outside.
It is an object of the present invention to provide such a heat exchanger which can be manufactured faster and at more reasonable cost.
It is also an object of the present invention to provide a manufacturing process for this type of a heat exchanger in which, for example, sheet metal sleeves become a component of the heat exchanger collecting pipe being manufactured, and interior and exterior surfaces thereof are absolutely gastight.
The foregoing objects have been achieved by connecting layers of sheet metal to form the collecting pipes which then consist either of a thin outer sheet metal sleeve with a material layer of the thickness of the collecting pipe wall applied on the inside or two concentric sheet metal sleeves between which a material layer is inserted.
It is an advantage of the present invention that the individual, relatively thin sheet metal layers can be provided with cutouts in a simple manner such as, for example, by mechanical punching, and subsequently, the individual sheet metal layers can be connected with one another with their cutouts aligned with one another. The individual machining of thin sheet metal layers in this case is significantly simpler than a machining of massive collecting pipes with a wall thickness which, depending on the overall size and the pressure difference, is between 5 and 15 mm.
When the cutouts of the thin sheet metal layers are mechanically punched, it is advantageously possible to avoid a sheet metal deformation, particularly with the cutouts located close together. It is also an advantage of the present invention that small heat exchanger tubes can be connected with the collecting pipes essentially without any warping.
According to one presently preferred embodiment of the present invention, each collecting pipe can comprise two axially joined half shells which are made of layers. The individual half shells can be made very easily by layering of sheet metal layers on one another. Compensation of moderately small tolerances with respect to the alignment of the cutouts can be made easily.
An alternative embodiment of the present invention provides that each collecting pipe is wound in several layers from a strip-shaped metal sheet. "Strip-shaped" means that the width of the strip corresponds to the length of the collecting pipe, while the strip length corresponds approximately to the collecting pipe circumference multiplied by the number of layers to be wound. The strip-shaped metal sheet, is provided advantageously before the winding operation either with a soldering layer or with a soldering foil with the same outside measurements wound between the sheet metal layers. The connection of the sheet metal layers in either case preferably takes place by a soldering-together. Thus, the solder is inserted between the individual sheet metal layers either as a solder foil with a thickness of approximately 3 to 10/100 mm, or the solder is applied directly to one or both sides of the metal sheets. After the sheet metal layers are placed on one another and are aligned, they are joined by being heated in a furnace.
The thickness of the sheet metal layers is preferably between 0.2 and 1 mm. A thickness 0.5 mm has been found to be particularly suitable because a mechanical punching-out operation can take place without any undesirable deformation while, at the same time, the number of sheet metal layers which have to be connected is not excessive. Any conventionally used steel or nickel alloy, such as Hastelloy X, C 263, X 10 CrNiTi 189, X 15 Cr Ni Si 2520, is suitable to be used for the sheet metal layers. Other deformable alloys are, however, also suitable for this purpose. 10 to 30 sheet metal layers for forming the collecting pipes can be arranged above one another and are connected. In such a case, it is most suitable for the sheet metal layers to have a thickness of 1/10 to 1/20 of the collector pipe wall to be manufactured.
The advantage of the embodiment of the present invention utilizing this outer sheet metal sleeve with a material layer applied on the inside is that cutouts in the sheet metal outer shell are easy to produce. As a result, a significant shortening of the production process can also be achieved. The application of the material layer from the interior to the sheet metal outer sleeve can be carried out with high pressure by suitable programmed coating machines within a relatively short time with the small streamlined-section tubes situated relatively close to one another. Suitable coating processes are, for example, plasma spraying or plast spraying processes. During the applicating operation, the small streamlined-section tubes are preferably closed on the inside by stoppers in order to prevent the penetration of layer material into the interior of the small streamlined-section tubes and then are subsequently removed.
This embodiment utilizing two concentric sleeves with a material layer inserted therebetween provides a simple process for making the cutouts in the collecting pipes.
A presently preferred process for manufacturing a heat exchanger in accordance with the present invention includes bending sheet-metal sheets with an individually required radius, punching in cutouts for small streamlined-section tubes, providing a solder layer on at least one side of the sheets, stacking and aligning the sheets with respect to the cutouts, fitting the section tubes into the cutouts, and solder connecting the metal sheets.
An essential advantage of the above-described manufacturing process resides in the fact that the joining of the individual sheet metal layers takes place with a heat source arranged inside the collecting pipe being manufactured. The interior wall of the collecting pipe has a higher temperature than the exterior wall, and thus the thermal expansions are higher on the inside than on the outside which results in a connection of the sheet metal layers layered above one another without any gaps. Hence, no undesirable hollow spaces remain between the sheet metal layers. It is also an advantage of the process of the present invention that the relatively thin-walled small streamlined-section tubes outside the collecting pipe are not directly stressed by the high soldering temperature.
Another advantageous process for manufacturing a heat exchanger collecting pipe in accordance with the present invention involves winding a sheet metal strip of the required width onto a rotatable winding pin, along with a solderable foil, and punching cutouts from the interior of the winding pin through the strip against a counterpart mounted outside the wound sheet.
An essential advantage of this embodiment of the process is that very thin sheet metal layers in the form of foils can be used which have a thickness of up to 0.2 mm. The costs for punching-out of the streamlined-section tubes are thus low. A significant simplification of the process is also achieved in that, at the same time as the sheet metal strip is wound, a soldering foil can be wound in with it, and by means of suitable devices in the interior of the collecting pipe, during each turn, the cutouts may be punched into the outermost sheet metal layer.
An advantageous further development of the described manufacturing process is that the cutouts of the sheet metal layers disposed above one another are manufactured in a geometrically similar manner from the outside to the inside with increasing size such that the cutouts of the finished collecting pipes are constructed to be slightly conical. After the small streamlined-section tubes have been fitted into the cutouts, the remaining annular gaps between the small streamlined-section tubes and the collecting pipe wall are connected by a build-up welding process. This arrangement has the advantage that, on the one hand, high demands are not made on the tolerance of the cutouts being manufactured; as a result, the punching and manufacturing process may be simplified. Nevertheless, a firm and gastight joining becomes possible between the small streamlined-section tube and the collecting pipe.
In a preferred further embodiment of the present invention, an adjustable laser beam is directed and focused from the inside to the point of the collecting pipe being worked, and metal powder is sprayed onto this focusing point along with inert gas. "Adjustable" means in this instance that either the collecting pipe is fixed and the laser is aligned, for example, by way of adjustable mirrors and prisms, or the laser beam is aligned and the collecting pipe is connected with a robot which has the required axes of motion, for example, six axes of motion. Suitable inert gases are argon, CO.sub.2, helium, or other known inert gases.
An advantage of the immediately-above described laser process is that the application process can be carried out under atmospheric conditions, after only the point being worked has been brought to a high temperature, while the rest of the component remains cool and is therefore not susceptible to oxidation. The melt-on depth achieved in this case amounts to approximately 0.05-0.2 mm, so that warping is avoided in the component. The supply of inert gas keeps the the heated point free of oxygen so that there is no risk of oxidation. An alloyed powder consisting of Hastelloy X or C 263 with a particle size of approximately 60-120 .mu.m has been found suitable for use as the metal powder.